first of all you would have to buy a converter like this for example....
http://www.nav-tv.com then next you would need to buy a dvd player, and hook it up to through the converter. you can use a ps2, an xbox, or you can buy a car dvd player for like $150. i too have an 04 with navigation and have explored this option already....
here's a pic of what it'll look like
other converters
http://www.mobilevideo4less.com/
http://www.navwithtv.com/grey.htm
http://www.avelectronic.com/